1. Field of the Disclosure
The present invention relates generally to circuits that drive light sources. More specifically, the present invention relates to circuits that drive light sources that may include dimming circuitry.
2. Background
As alternatives to incandescent light bulbs become more affordable and increase in popularity, many traditional incandescent light bulbs are being replaced by alternative light sources. One example of an alternative light source is light emitting diode (LED) lighting. Many LED light sources are designed to be compatible with existing sockets that were originally designed to work with conventional incandescent light bulbs so that the LED light sources are “drop-in” replacements. To utilize the existing wiring, many ac-dc LED driver circuits are designed to operate and drive the LED light sources when the ac power to the LED driver circuits is controlled by a conventional light switch or a conventional dimmer.
Dimmers are used in a variety of residential and commercial applications to vary the brightness of lights. However, often dimmers are triac-based dimmers that function by varying the percentage of time or the portion of each ac half cycle of an ac input signal that is removed from an ac input signal supplying power to a light source. When triac-based dimmers remove portions of each ac half cycle of an ac input signal, sharp switching edges are generated. These switching edges create electromagnetic interference (EMI). EMI is a disturbance that interrupts radio, television and cell phone signals and presents an increasing problem as more and more devices (e.g. printers, cameras, headphones/headsets, computers, etc.) communicate wirelessly.
Triac-based dimmers also lower the power factor of the energy grid by distorting input current waveforms. Like EMI, power factor is an increasingly important aspect of lighting products being installed in residential and commercial lighting applications. A low power factor increases power loss and imposes additional infrastructure costs on power utility providers. Recognizing the size of these costs, legislation has placed requirements on power factor around the world.
Triac-based dimmers also present dimming range problems, especially for alternative light sources. Triac-based dimmers remove a large portion of each ac half cycle of an ac input signal when low light output is required. As a result of large portions being removed from the signal, light sources are starved for power, which tends to cause light flicker in low light output conditions.